1. Field of the Invention
The present invention relates to an amplifying circuit, and more specifically, to an amplifying circuit capable of having high equivalent input impedance, high voltage gain (or high voltage attenuation), or a large time constant.
2. Description of the Prior Art
In basic circuit structures, amplifying circuits play a very important role. An amplifying circuit can be classified for different uses such as signal amplifying or power amplifying according to its application, the most common of which is the signal amplifying circuit.
Please refer to FIG. 1 showing an example of a signal amplifying circuit using an operational amplifier. In the structure shown in FIG. 1, a formula as follows can be applied. Since the negative input end of the operational amplifier is virtually grounded, the voltage on the negative input end is 0V. Therefore, no current flows through the negative input end, and an equation I1+I2=0 can be found. As a result, the following formula can be found:Vo/Vi=−Z2/Z1  formula 1
In general application, in order to obtain better signal quality and frequency response, an amplifying circuit having high equivalent input impedance, high voltage gain (or high voltage attenuation), or a large time constant is preferred. Thus different resistive impedances, capacitive impedances or inductive impedances are usually installed in the places of the impedance Z1 and impedance Z2 and arranged in different manners in order to achieve the above-mentioned objective.
However, passive elements such as resistors, capacitors and inductors used to achieve the preceding goal require very high values and therefore consume a very large circuit area when manufacturing, which causes a large cost in integrated circuit manufacturing.